1. Field of the Invention
The invention is directed to a hydraulic system for controlling the contact force applied by a hydraulic device. The system is provided with a pilot-controlled main valve and a solenoid-controlled pilot valve. The main valve can be moved by means of the pilot valve from a position that supplies pressurized fluid to a hydraulic device to a position that drains pressurized fluid from the hydraulic device.
2. Description of the Prior Art
Hydraulic systems to control the position or the contact pressure of a hydraulic device are widely known. Some of these systems are used to control the distance of an agricultural implement, such as a harvesting platform, from the ground.
Pressurized fluid can be supplied to lifting cylinders through a main valve (TM-1352, page 270-20-5 and 270-20-7 through 270-20-9, Mar-86, by John Deere), when this is switched from a blocking position to a through position. The switching is accomplished with a pilot valve, that is remotely controlled and, depending on the control operation, reduces the pilot pressure behind the main valve, so that it is moved to the through position by means of a pressure applied to the other side of the main valve. This hydraulic system provides only a controlled lifting or lowering of an implement without any automatic position control of the implement.
According to another control scheme (Bosch--Hydraulics, System Information, Circuitry and Components for Combines, K6/VKD 2-BEy 017/5 De (5.04), pages 4 and 5) a manually-operated three-way valve is followed by two two-way valves and one adjustable pressure relief valve, where the pressure relief valve and a two way valve are required to move the other two-way valve from a blocking position to a through position upon reaching a pre-set pressure. The cost of this system in terms of valves and control connections is relatively expensive.